Dragonfly predators influence biomass and density of pond snails

Abstract

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.

Interactions among invaders: community and ecosystem effects of multiple invasive species in an experimental aquatic system

“…The experiment was conducted over an 8-week period in 24 outdoor mesocosms (1,200 l) randomly assigned to one of the following treatments: (1) native snails only, (2) native snails with Bellamya, (3) native snails with Orconectes, and (4) native snails with both invaders (six replicates per condition). Mesocosms were made of structural foam plastic and measured 1.6 9 1.7 m with a fill height of 0.5 m. While not equivalent to ecosystem-level manipulations, mesocosm studies can provide important insights about mechanistic interactions in aquatic food webs, including snail-predator interactions (e.g., Nyström et al 1999;Turner and Chislock 2007). Prior to initiating the experiment, we seeded each mesocosm with *1,000 l lake water, 18 kg commercial ''play sand'' and 27 kg coarse (1.9 cm diameter) gravel as substrate, two cinderblocks as refugia, and 28 g CaCO 3 to ensure sufficient Ca for shell growth.…”

Ecological Drivers of Cladoceran Diversity in the Central European City (Łódź, Poland): Effects of Urbanisation and Size of the Waterbody

“…Invertebrate densities in submerged macrophyte beds were not influenced by fish indicating that macrophytes can act as refugia for invertebrate taxa (Gilinsky, 1984;Diehl, 1992). Within fishless ponds, Dytiscidae, Hemiptera and Odonata are top predators and can influence community structure by reducing macroinvertebrate richness and abundance at both local and metacommunity scales (Cadotte et al, 2006;Turner and Chislock, 2007;Cobbaert et al, 2010).…”

The distribution of pond snail communities across a landscape: separating out the influence of spatial position from local habitat quality for ponds in south-east Northumberland, UK

“…Local factors may also include interactions with other species such as predation (e.g. by Odonata, Turner and Chislock, 2007), or physiological responses e.g. impacts from toxic cyanobacteria, (Gerard et al, 2008).…”